Yesterday I read an interesting paper
Enol Tautomers of Watson−Crick Base Pair Models Are Metastable Because of Nuclear Quantum Effects
Alejandro Pérez, Mark Tuckerman, Harold Hjalmarson, and Anatole von Lilienfeld
A key to the double helix structure of DNA and its ability to provide reliable stable storage of genetic information is hydrogen bonding between base pairs [G-C and A-T].
However, it is possible to switch around the positions of the protons on each of the base pairs, producing different tautomers of T, A, C, and G].
This is an example of double proton transfer.
An important question concerns just how rare this is. For example, what is the free energy of these tautomers relative to the Watson-Crick ones?
Over the past two decades a number of classical molecular dynamics simulations, using potentials derived from quantum chemistry suggested that the tautomers of DNA could be a problem.
More than 50 years ago Lowdin [of natural orbital fame] considered the role that quantum tunneling of protons may have an important role on mutations.
Well, the paper above actually shows/argues [based on ab initio path integral molecular dynamics] that the quantum motion of the protons destabilises the mutant tautomers.